The present invention relates to a sol-gel process for the preparation of vitreous films characterized by high adhesion on the substrate of interest, to the vitreous films obtained thereby, and to the colloidal suspensions obtained in the initial phase of the above mentioned process, that can be removed and maintained as such for indefinite amount of time, before being processed again to achieve the film deposition.
More particularly, the object of the present invention is a process for the preparation and the deposition, on the surface of suitable substrates, of vitreous films comprising the operations of dissolving one or more metal alkoxides in an aprotic solvent, of adding controlled amounts of an aqueous solutions of a catalyst to the solution thereby obtained, of monitoring with adequate precision the hydrolysis reaction, of the eventual extraction and removal the amount of alcohol produced in the hydrolysis reaction, of the gelation of the sol on the surface of interest and of the final drying of the film. It is of particular importance in this invention, the possibility to stop the process after hydrolysis and before film deposition to isolate a sol that in the apparent condition of a clear and stable solution, can be stored safely for long time at room temperature.
The vitreous films are used in the field of telecommunications on physical carriers for both optical and electronic applications; because of their remarkable resistance to temperature, abrasion and corrosion, silica films are used, as an example, in processes typical of the electronic industry as surface planarizers in a semiconductor after previous treatments have created off-plane circuitry. Similar silica films are also used to produce electrical insulation between intracircuitry elements or between conductors of different circuits on a semiconductor chip.
Moreover [A. S. Holmes et al., “Applied Optics”, Jan. 9, 1993, vol. 32, n. 25, pages 4916 -4912], silica films find applications as antireflexion and as planar wave-guides, or even as host material for active dopants for nonlinear optic as well as optical sensing applications.
Many methods were developed for the preparation of optical devices [A. S. Holmes et al., ibidem], among other, as an example, can be called the thermic oxidation of silicon, or the technique of deposition under reduced pressure, known as “sputtering”, chemical vapor deposition, etc.
The technologies to which the above-cited methods refer are, however, relatively complex and sophisticated, their practical exploitation require specialized equipment and time-consuming procedures that result in relatively costly operations.
Moreover, the limited productivity to which often they are coupled erodes the convenience for their industrial application.
It is a consolidated conclusion [A. S. Holmes et al., ibidem; Rui M. Almeida, “International Journal of Optoelectronics”, 1994, vol. 9, n. 2, pagg. 135-142], that the more promising technology for large scale production of vitreous films is the sol-gel technology. Such a technology is based on three fundamental steps:    a. sol formation in alcoholic medium (particle suspension or dispersion in a liquid),    b. gelation or transformation of sol by condensation into gel (a solid skeleton inglobating a continuous liquid phase),    c. drying of the gel.
In the procedure for vitreous film deposition on substrate, the sol is applied to the surface to be coated and the gel is formed as consequence of the solvent evaporation; a drying phase by simple heating complete the coating procedure.
It is however known that in the production of vitreous films, even through sol-gel based technology one have to face limitations, specially in the thickness of the films obtained and/or in the tendency of such films to crack, often in the drying phase.
Solutions to the problem were promptly sought and some of technical answers reported in this field by the scientific literature and by recent patents, with no doubt contain preludes to future industrial application. Examples of such important reports are following:                Italian patent application n. NO 98 A 000004 describes a sol-gel process for the preparation of thick films of silicon-dioxide that improves the well-known procedure by the addition of a certain quantity of fumed silica to the hydrolysis product of silicon alkoxide.        A. S Holmes et al. already quoted article describes a process of high-temperature consolidation of a multilayers film.        U.S. Pat. No. 6,130,152 describes a sol-gel process that teaches to add a combination of two solvents with different boiling point to the hydrolyzed solution (tetraethylorthosilahe, ethanol, water and acid).        U.S. Pat. No. 6,017,389 describes the preparation of siliceous films starting from the combination of tetraethylorthosilane and silica in anhydrous ethanol that is hydrolyzed by aqueous ammonia and necessitate of a very high thermal treatment at the end.        
All the known solutions, of which the precedent paragraphs are only a few examples, make the sol-gel technology attractive for the preparation of vitreous films, but do not offer a fully convenient process for a broad industrial application either because of the temperature requirements or because of thickness, or for particular technicalities that could negatively influence the industriability of the process.
The Applicant has now found that it is possible to prepare and to deposit on substrates, according to the sol-gel technology, vitreous films following a procedure presenting none of the disadvantages of the known art, nor the limitations to general applicability so far reported on count of sol-gel technology.